The realization of long-lasting and rapidly rechargeable electrical energy storage is perhaps the most important barrier to the replacement of carbon-based fuels by electrical energy systems.

This project aims to develop a practical electrical energy storage device that combines the long life and rapid charge-discharge capability of a capacitor with the much higher energy storage capacity of a rechargeable battery. We believe that this can be achieved by increasing the energy storage capacity of a commercial electrical energy storage device, called a double layer capacitor (DLC) or ultracapacitor, by replacing the activated carbon electrode coating with an array of vertically-aligned nanotubes. The predicted performance of the nanotube-enhanced ultracapacitor using the vertically-aligned nanotube array is significantly higher (30 to 60 Wh/Kg) than conventional batteries because of its improved electrical characteristics and because the diameter and spacing of the nanotubes are matched to the dimensions of the electrolyte ions, in contrast to the irregular pores exhibited by activcated carbon.

Fig. 7. (below, left). Diagram of Nanotube-Enhanced Ultracapacitor Configuration (not to scale - nanotubes are much longer than shown). Top and bottom ends are the electrodes. Vertical tubes are the nanotubes. Circles are the ions. + and – signs represent positive and negative charges.
Fig. 8. (below, right). Electrode cross-section. Bottom plate is the electrode, vertical “hairs” are the nanotubes.Joel Schindall.

Traditionally, efforts to improve power- and energy-density in fuel-cells and batteries have focused on developing new materials for electrodes, catalysts, and membranes. However, in efforts to control device costs, advanced nanofabrication, processing, and lithographic methods have not been used to optimize the cell structure. In recent years, with the advent of techniques of nanoimprint, roll-to-roll printing, and other low-cost nanofabrication methods, top-down "lithographic" design and development of integrated battery and fuel-cell structures can be envisaged and realized. We are working to combine these methods with nanopatterned electrodes and membranes for fuel cells and batteries, in an effort to improve the efficiency, lower the cost, and thus ultimately change the way energy is stored and used in the world. See: http://quantum.mit.edu/.

Fig. 9. Nanopatterned electrodes promise to improve the efficiency of both fuel cells and batteries.